Advances have been made in recent years in the design of structural components for the aerospace industry, but the provision of lightweight, reliable joint connections between such structural components continues to present problems. One type of joint connection which has been used in the past is fasteners in the form of bolts, rivets, screws and the like. One problem with fasteners is that they require holes to be formed in the structural components to be joined. As a result, the adjoining portions of the structural components must be made somewhat larger to make up for the reduction of load carrying capability lost by the formation of holes therein. Such holes also tend to create stress concentrations in the structure adjacent the holes which can result in the formation of a fatigue crack thereat. Additionally, the presence of fasteners between adjoining structural components adds weight to the joint which is undesirable in any aerospace application.
In view of the problems with fasteners, efforts had been made to join abutting structural components by creating a bond therebetween such as by adhesive bonding, brazing, welding and diffusion bonding. While some of these techniques solve many of the problems associated with the use of fasteners, other problems are created, particularly in applications such as the construction of jet aircraft which operate at relatively high, sustained temperatures.
For example, most organic adhesives cannot be used to bond components subjected to the types of operating temperatures in the turbo-machinery of jet aircraft. Braze metals have been proposed as an alternative to organic adhesives, but such metals are also affected by heat. If a braze metal connecting two structural components is maintained at a melting temperature for too long, it may tend to flow away from the brazed area causing failure of the joint. At least some braze metals tend to dissolve into the metal of the adjoining structural components to be connected, and form an embrittled alloy having reduced strength.
One alternative to adhesive bonding, welding or brazing is diffusion bonding, which has been used effectively in the attachment of some titanium components. Although not precisely understood, it is believed that the process of diffusion bonding operates by bringing the surfaces of two components into contact with one another at sufficient heat and pressure so that the atoms adjacent the adjoining surfaces move together to- form atomically or molecularly mating surfaces which hold the components together as a unitary structure. As disclosed, for example, in U.S. Pat. No. 3,633,267, "honeycomb" structures or panels have been successfully formed using diffusion bonding techniques. These honeycomb panels comprise a titanium honeycomb or cellular core which is diffusion bonded to opposed titanium face sheets, and, in some configurations, to opposed side sections. Such panels have been used extensively in the aerospace industry because of their favorable strength to weight ratio.
While diffusion bonding has been successfully employed to form the honeycomb panels themselves, a more difficult problem is presented in bonding structural components to a honeycomb panel. As described above, the process of diffusion bonding requires the application of sufficient pressure and heat to the surfaces to be bonded to cause the atoms or molecules of the adjoining surfaces to move together. It has been found that the application of a relatively large compressive force on a honeycomb panel can cause damage to its cellular core. As a result, honeycomb panels have been bonded to one another using additional face sheets, plates and/or fasteners. The use of fasteners creates a number of problems as discussed above, and additional face sheets or plates increases the weight of such components.
Improved methods of diffusion bonding have been proposed which protect the cellular core of honeycomb panels while obtaining the desired diffusion bond between the surface of a honeycomb panel and an abutting structural component. U.S. Pat. No. 4,013,210, for example, discloses a method of diffusion bonding honeycomb panels to other structural components in which a substantial vacuum is applied at the surfaces wherein the diffusion bonding takes place. Under this vacuum, the surfaces are heated and brought into contact with one another with only moderate pressure being required to effect the diffusion bond.
In order to apply the required compressive force to the honeycomb panel and/or structural components to be bonded, the method disclosed in the U.S. Pat. No. 4,013,210 employs a glass pad positioned against a face sheet of the honeycomb panel which, when melted under diffusion bonding temperatures, is sufficiently compliant to evenly transfer compressive force over the entire face sheet. This compressive force is applied to the glass pad by a weight which is initially spaced from the glass pad by glass spacer blocks. At diffusion bonding temperatures, these glass spacer blocks melt and permit the weight to move against the glass pad which applies a compressive force therethrough to the face sheet of the honeycomb panel. In turn, the honeycomb panel is urged against a structural component to be bonded thereto with moderate pressure which protects the core of the honeycomb panel from damage.
One problem with the diffusion bonding method disclosed in U.S. Pat. No. 4,013,210 is that the application of a compressive force through the glass pad is dependent upon the movement of a weight thereagainst. The weight is supported on at least two glass blocks which must melt at the same rate to permit the weight to uniformly move against the glass pad on the surface of the honeycomb panel. This movement of the weight is substantially uncontrolled once the diffusion bonding process begins, and it is contemplated that the weight could become misaligned or tipped with respect to the glass compliant pad and honeycomb panel which would result in the application of an uneven compressive force across the honeycomb panel.